1. Field of the Invention
The present invention relates to a variable capacitance device using a piezoelectric film and a portable phone including the variable capacitance device.
2. Related Art
In recent years, radio communication techniques have achieved remarkable progress and are still under development for transmitting information at higher speeds. In the market of radio communication techniques, frequency bands around 2 GHz have been widely used in response to the introduction of the Personal Handyphone System (PHS), third-generation mobile communications, wireless LANs and so on, and the number of subscribers and terminals has dramatically increased. In order to transmit information at higher speeds, higher carrier frequencies are used and the commercialization of frequency bands up to 5 GHz has been started in wireless LAN systems.
Regarding such high-frequency communications equipment, reduction of size and weight is strongly demanded. Particularly, for use in personal computers (PCs), it is highly important to fabricate communications equipment with a small thickness for use as a PC card.
Generally, wireless equipment such as a PC card is broadly divided into an RF front end unit for processing radio frequencies (RF) and a baseband (BB) unit for processing digital signals. Of these units, the BB unit modulates and demodulates signals through digital signal processing. Basically, the BB unit can be configured using an LSI chip including a Si substrate and thus the height of the BB unit can be easily reduced to about 1 mm or less.
On the other hand, the RF unit performs amplification, frequency conversion and so on while using a radio frequency signal as an analog signal. The RF unit is hard to configure only with an LSI chip, and thus the RF unit has a complicated configuration including a number of passive components such as a resistor, a capacitor, an inductor, an oscillator, and a filter.
Semiconductor parts serving as LSI chips have been widely developed mainly with the aim of achieving finer design rules. While LSI chips have been reduced in size with greater functionality, it is difficult to omit the passive components of the RF unit because each of the passive components has functions determined by specifications. Further, the passive components are hard to fabricate with semiconductors, and thus the passive components are fabricated separately from semiconductor chips and then assembled with the semiconductor chips on semiconductors or another substrate in many cases.
However, such passive components have been also miniaturized and the number of passive components has increased more and more, causing a serious problem on an assembly unit for achieving high-density packaging or a throughput of an assembly. Although the above explanation has been described as an example of wireless communications, this problem arises not only in the field of wireless communications but also in all kinds of electronic components and electronic devices.
In order to solve this problem, in recent years, considerable attention has been focused on techniques of fabricating switches and varicaps by means of actuators fabricated by a technique of Micro-electro-mechanical System (MEMS) (See U.S. Patent No. 2004/150939). Particularly, for high-frequency use including portable phones and car phones, MEMS switches achieve a low loss and high insulation during an off period as compared with conventionally used semiconductor switches and varicaps. Further, MEMS varicaps achieve a high Q value. For these reasons, the use of such MEMS switches and varicaps is regarded as promising.
A MEMS varicap includes, for example, a movable electrode which is provided on a beam of an actuator having one end supported in the air above a substrate, and a fixed electrode which is provided on a surface of the substrate facing the actuator. A capacitance between electrodes is varied by changing a distance between the movable electrode and the fixed electrode by means of the actuator.
A MEMS switch can be configured by using the structure of the MEMS varicap as it is. Alternatively, a MEMS switch can be configured by directly contacting a movable electrode and a fixed electrode with each other or causing the movable electrode and the fixed electrode to face each other via quite a thin dielectric film in a high frequency region of GHz or higher.
As a driving mechanism for a MEMS actuator, there are methods in which a beam is displaced by bending with an electrostatic force, a thermal stress, an electromagnetic force, a piezoelectric force and the like. In the case of electrostatic driving, although the power consumption is low, a high voltage of about 10 V to 100 V is required for driving an actuator. Further, in an electrostatically driven varicap, when a distance between a movable electrode and a fixed electrode exceeds one third of an original distance between the electrodes, the movable electrode is pulled in the fixed electrode, so that a rate of change in capacitance cannot be high. In thermal driving and electromagnetic driving, current passes through a resistor and an electromagnetic coil and thus power consumption increases.
On the other hand, in the case of piezoelectric driving, the power consumption is low, an operation is enabled at a driving voltage of 10 V or less, and a movable electrode is not pulled in a fixed electrode. Thus, it is possible to greatly change an actuator in a continuous manner and a rate of change in capacitance can be high. As described above, a piezoelectric driving mechanism has a number of advantages as compared with the other driving mechanisms and attracts attention as a MEMS switch and varicap.
However, since a piezoelectric driving actuator has a long and thin beam structure including a piezoelectric film interposed between upper and lower electrodes, the beam warps upward and downward due to a small residual stress of a material of the piezoelectric film or the upper and lower electrodes. Further, in the case where a switch or a varicap is formed using a part of the upper and lower electrodes of a piezoelectric actuator as a movable electrode, in order to prevent a signal transmitted from the fixed electrode to the movable electrode from passing through a piezoelectric driving power supply via the upper and lower electrodes, a slit region including no electrodes is formed between the upper and lower electrodes and the movable electrode to provide electrical isolation. The region including no electrodes on the piezoelectric film also warps due to a residual stress, as on the actuator. However, an amount of warp is different between the region and the actuator by whether the electrodes are present or not. For this reason, even when the residual stress of the piezoelectric film and the upper and lower electrodes is optimized to eliminate the warp of the actuator, a warp occurs from the boundary of the upper and lower electrodes and the movable electrode.
In order to solve this problem, it is important to reduce the residual stress of the piezoelectric film. Film-forming apparatuses have actually improved and technical development is obtained for reducing a residual stress without causing problems under operating conditions. However, variations are actually found in mass production. Thus, it is quite difficult to stably comply with a specification value required in a system specification.
Moreover, when this kind of piezoelectric driving actuator is applied to a portable phone and the like, a serious and potential problem arises as follows. When the portable phone is dropped and hit on the ground, a piezoelectric bridge is also accelerated, the operation becomes unstable, and the bridge is brought into contact with an opposing electrode and degraded. Further, portable phones have complicated structures and a space for storing the piezoelectric driving actuator is limited.